Documentation

TauCeti.Analysis.Holder.Normed

The Banach space of global Hölder functions #

This file equips bounded continuous Hölder functions with the norm

‖f‖_[C^α] = ‖f‖_∞ + [f]_α

and proves that this norm is complete when the codomain is complete. The supremum term controls the pointwise limit of a Cauchy sequence, while the Hölder term controls its increments uniformly. Thus the limiting function is again Hölder and convergence holds in both terms of the norm.

The resulting Banach space is the zeroth-order member of the C^{k,α} scale used for Schauder estimates. The definition uses Mathlib's MemHolder and nnHolderNorm rather than introducing a parallel notion of Hölder continuity.

Main declarations #

References #

L. C. Evans, Partial Differential Equations, Section 5.1; D. Gilbarg and N. Trudinger, Elliptic Partial Differential Equations of Second Order, Section 4.1.

structure TauCeti.HolderSpace (α : NNReal) (X : Type u) (Y : Type v) [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] :
Type (max u v)

The space of bounded continuous globally α-Hölder functions, equipped below with the supremum-plus-Hölder norm. The wrapper separates this norm from the inherited supremum norm on the underlying submodule.

  • toHolderSubmodule : ↥(holderSubmodule α)

    The underlying bounded continuous Hölder function.

Instances For
    @[instance_reducible]
    instance TauCeti.HolderSpace.instCoeFunForall {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] :
    CoeFun (HolderSpace α X Y) fun (x : HolderSpace α X Y) => X → Y
    Equations

    The underlying bounded continuous function.

    Equations
    Instances For

      Promote a bounded continuous Hölder function to the Hölder space.

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      Instances For

        Every element of the bounded Hölder space is continuous, including at exponent zero.

        A Hölder-space element satisfies the global Hölder condition.

        theorem TauCeti.HolderSpace.ext {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] {f g : HolderSpace α X Y} (h : ∀ (x : X), ↑f.toHolderSubmodule x = ↑g.toHolderSubmodule x) :
        f = g

        Two Hölder-space elements are equal when they agree pointwise.

        theorem TauCeti.HolderSpace.ext_iff {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] {f g : HolderSpace α X Y} :
        f = g ↔ ∀ (x : X), ↑f.toHolderSubmodule x = ↑g.toHolderSubmodule x
        @[instance_reducible]
        Equations
        @[instance_reducible]
        Equations
        @[instance_reducible]
        noncomputable instance TauCeti.HolderSpace.instNeg {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] :
        Neg (HolderSpace α X Y)
        Equations
        @[instance_reducible]
        noncomputable instance TauCeti.HolderSpace.instSub {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] :
        Sub (HolderSpace α X Y)
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        @[instance_reducible]
        Equations
        @[instance_reducible]
        Equations
        @[instance_reducible]
        noncomputable instance TauCeti.HolderSpace.instSMulInt {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] :
        Equations
        @[simp]
        theorem TauCeti.HolderSpace.zero_apply {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] (x : X) :
        ↑(toHolderSubmodule 0) x = 0
        @[simp]
        theorem TauCeti.HolderSpace.add_apply {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] (f g : HolderSpace α X Y) (x : X) :
        @[simp]
        theorem TauCeti.HolderSpace.smul_apply {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] (c : ℝ) (f : HolderSpace α X Y) (x : X) :

        The underlying bounded continuous function determines a Hölder-space element.

        @[instance_reducible]
        noncomputable instance TauCeti.HolderSpace.instModuleReal {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] :
        Equations
        @[instance_reducible]
        noncomputable instance TauCeti.HolderSpace.instNorm {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] :
        Norm (HolderSpace α X Y)

        The supremum-plus-Hölder norm, combining uniform size with the global Hölder seminorm.

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        The Hölder-space norm is the supremum-plus-Hölder norm of the underlying bounded continuous function.

        The Hölder-space norm is the sum of the supremum norm and the global Hölder seminorm.

        @[instance_reducible]

        The supremum-plus-Hölder norm makes HolderSpace α X Y a normed additive commutative group.

        Equations
        @[instance_reducible]
        noncomputable instance TauCeti.HolderSpace.instNormedSpace {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] :

        The supremum-plus-Hölder norm makes HolderSpace α X Y a normed space over ℝ.

        Equations

        The supremum norm is controlled by the Hölder-space norm.

        Forgetting the Hölder bound is a continuous linear map to bounded continuous functions.

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        Instances For

          Forgetting the Hölder bound has operator norm at most one.

          noncomputable def TauCeti.HolderSpace.evalCLM {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] (x : X) :

          Evaluation at a point as a continuous linear map on the Hölder space.

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            Evaluation factors through the inclusion into bounded continuous functions.

            @[simp]
            theorem TauCeti.HolderSpace.evalCLM_apply {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] (x : X) (f : HolderSpace α X Y) :

            Evaluation has operator norm at most one.

            Bounded global α-Hölder functions form a Banach space when the codomain is Banach.

            The Hölder seminorm is controlled by the Hölder-space norm.

            def TauCeti.HolderSpace.const {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] (c : Y) :

            A constant function as an element of the bounded Hölder space.

            Equations
            Instances For
              @[simp]
              theorem TauCeti.HolderSpace.const_apply {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] (c : Y) (x : X) :

              The Hölder norm of a constant is at most the norm of its value, even on an empty domain.

              @[simp]

              On a nonempty domain, constant functions have exactly the norm of their value.

              @[simp]
              theorem TauCeti.HolderSpace.const_add {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] (c d : Y) :
              const (c + d) = const c + const d
              @[simp]
              theorem TauCeti.HolderSpace.const_smul {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] (c : ℝ) (d : Y) :
              const (c • d) = c • const d
              noncomputable def TauCeti.HolderSpace.constL {α : NNReal} {X : Type u} {Y : Type v} [MetricSpace X] [NormedAddCommGroup Y] [NormedSpace ℝ Y] :

              The continuous linear map assigning a constant bounded Hölder function to each value.

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              Instances For
                @[simp]

                The constant map has operator norm at most one, including on an empty domain.

                @[simp]

                On a nonempty domain with nontrivial values, the constant map has operator norm one.

                @[simp]

                On a nonempty domain with nontrivial values, the inclusion has operator norm one.

                @[simp]

                With nontrivial values, evaluation has operator norm one.